Optical fibers typically include an outer polymer coating to protect the core/cladding glass portions. The light transmitting performance of an optical fiber is highly dependent upon the properties of the polymer coating that is applied to the fiber. Typically a dual-layer coating system is used. The dual-layer coating includes a softer (lower modulus) primary coating in contact with the glass fiber and a harder (higher modulus) secondary coating that surrounds the primary coating. The harder secondary coating allows the fiber to be handled and further processed, while the softer primary coating plays a key role in dissipating external forces and preventing them from being transferred to the fiber where they can cause microbend induced light attenuation.
The functional requirements of the primary coating place various constraints on the materials that are used for these coatings. The Young's modulus of the primary coating is generally less than 1 MPa, and is ideally less than 0.5 MPa. The glass transition temperature of the primary coating is less than 0° C., and is ideally less than −20° C. to ensure that the primary coating remains soft when the fiber is deployed in a low temperature environment. In order to ensure uniform deposition on the fiber, the primary coating is applied to the fiber in the form of an uncured liquid composition and must quickly cure to form a solid coating having sufficient mechanical integrity to support subsequent application of the liquid composition used to form the surrounding secondary coating. Also, the tensile strength of the primary coating, which generally decreases as the modulus decreases, must be high enough to prevent tearing defects during on draw processing or subsequent processing of the coated fiber during cabling, etc.
In order to meet these requirements, optical fiber coatings are usually formulated as mixtures of radiation curable urethane/acrylate oligomers and radiation curable acrylate functional diluents. Upon exposure to light in the presence of a photoinitiator, the acrylate groups rapidly polymerize to form a crosslinked polymer network, which is further strengthened by hydrogen bonding interactions between urethane groups along the oligomer backbone. By varying the urethane/acrylate oligomer, it is possible to form coatings having very low modulus values while still maintaining sufficient tensile strength. Numerous optical fiber coating formulations have been disclosed in which the composition of the radiation-curable urethane/acrylate oligomer has been varied to achieve different property targets.
Radiation-curable optical fiber coatings having low modulus values and low glass transition temperatures can be prepared using acrylate functional oligomers alone, such as polyalkylene glycol diacrylates, but such coatings typically have very poor tensile strength due to the absence of the reinforcing urethane groups found in the more commonly used coatings. The use of a non-radiation-curable thermoplastic elastomer as a toughening additive in a crosslinked radiation curable all acrylic optical fiber coating has been disclosed in U.S. Pat. No. 6,810,187. Specifically disclosed are block copolymers comprising a thermoplastic polyurethane, styrene butadiene, EPDM, ethylene propylene rubber, synthetic styrene butadiene rubber, styrenic block compolymers or combinations, where the elastomeric soft block comprises a polybutadiene, hydrogenated polybutadiene, polyisoprene, polyethylene/butylene, polyethylene/propylene, diol block or combinations thereof.
However, the nature of these thermoplastic elastomers may limit their solubility in, and consequently their ability to toughen, a typical fiber coating composition based on acrylic monomers. Solid, high molecular weight thermoplastic urethane elastomers are insoluble or sparingly soluble in most acrylic monomers. The poor solubility limits the amount of thermoplastic elastomer that can be used as a toughening additive in a coating formulation. Slightly higher solubility of thermoplastic urethane elastomers is observed in highly polar acrylic monomers, but such monomers are expensive and the solubility remains well below the levels desired for a practical coating composition. Many highly polar acrylic monomers are also known to cause excessive smoking during the coating operation when drawn on optical fiber. Other elastomer additives, such as those based on butadiene or other hydrocarbon-like soft blocks, are only soluble in highly non-polar monomers, such as lauryl acrylate, isodecyl acrylate or tridecyl acrylate, which are known to inhibit fiber coating curing speeds. Also, the increase in coating viscosity resulting from addition of larger amounts of a high molecular weight elastomer with only limited solubility in the coating monomer is often detrimental to the coating operation.
There remains a need for coating formulation additives that enable economical primary coating materials that possess low modulus and high tensile strength.